A dual connectivity framework is currently being considered for LTE Rel-12. Dual Connectivity refers to the operation where a given UE consumes radio resources provided by at least two different network points (Master eNB, MeNB and Secondary eNB, SeNB) connected with non-ideal backhaul while in RRC_CONNECTED. A UE in dual connectivity maintains simultaneous connections to anchor and booster nodes, where the MeNB is interchangeably called as anchor node and the SeNB is interchangeably called as the booster node. As the name implies, the MeNB controls the connection and handover of SeNB so signaling in MeNB is needed even if SeNB changes. In addition, both the anchor node and booster node can terminate the control plane connection towards the UE and can thus be controlling some of the UE operations independently with respect to termination of the control plane.
The UE reads system information from the anchor node. In addition to the anchor node, the UE may be connected to one or several booster nodes for added user plane support. The MeNB and SeNB are connected via the Xn interface, which is currently selected to be the same as the X2 interface between two eNBs.
Dual connectivity (DC) is a mode of operation of a UE in for example RRC_CONNECTED state, where the UE is configured with a Master Cell Group (MCG) and a Secondary Cell Group (SCG). Cell Group (CG) is a group of serving cells associated with either the MeNB or the SeNB. The MCG and SCG are defined as follows:                Master Cell Group (MCG) is a group of serving cells associated with the MeNB, comprising of the PCell and optionally one or more SCells.        Secondary Cell Group (SCG) is a group of serving cells associated with the SeNB comprising of PSCell (Primary Scell) and optionally one or more SCells        
Master eNB is the eNB which terminates at least S1-MME. Secondary eNB is the eNB that is providing additional radio resources for the UE but is not the Master eNB.
FIG. 1 describes a dual connectivity setup. In this example, only one SeNB is connected to UE, however, more than one SeNB can serve the UE in general. As shown in the figure, it is also clear that dual connectivity is a UE-specific feature and a network node can support a dual connected UE and a legacy UE at the same time.
As mentioned earlier, the anchor node and booster node roles are defined from a UE point of view. This means that a node that acts as an anchor node to one UE may act as booster node to another UE. Similarly, though the UE reads the system information from the anchor node, a node acting as a booster node to one UE, may or may not distribute system information to another UE.
The MeNB may provide system information, terminate control plane and it can also terminate user plane
The SeNB, an terminate the control plane but can only terminate the user plane
In one application, dual connectivity allows a UE to be connected to two nodes to receive data from both nodes to increase its data rate. This user plane aggregation achieves similar benefits as carrier aggregation using network nodes that are not connected by low-latency backhaul/network connection. Due to this lack of low-latency backhaul, the scheduling and HARQ-ACK feedback from the UE to each of the nodes will need to be performed separately. That is, it's expected the UE shall have two UL transmitters to transmit UL control and data to the connected nodes.
In some examples, the anchor node provides a macro cell while the booster cell provides a lower power (e.g., micro, pico, or femto) cell (at least partially within the coverage area of the macro cell). For instance, the UE connection may be anchored to the macro cell via an anchor carrier while data rate is boosted via a lower power cell through a booster carrier. In this other similar cases, dual connectivity may be termed dual-layer connectivity since the UE has simultaneous connections to both macro and low-power layers. Regardless, the macro cell may provide system information and/or basic RRC signaling (such as paging or mobility related signaling), with all data services (but no system information) being provided by the low-power cell (i.e., the low-power cell is “macro assisted”) for that particular UE. In a different use case, though, the macro cell may also provide at least some data services (e.g., lower rate or high reliability data services), whereas the low-power cell provides higher rate data services.
In dual connectivity the UE will be connected to two eNodeBs simultaneously; MeNB and SeNB. Each of them may have one or more associated SCells which may be configured for DL, or DL and UL CA operation. The SCells are time-aligned to the MeNB and SeNB, respectively, but the MeNB and SeNB may or may not be time aligned with respect to their frame timings and/or their respective system frame number (SFN).
MeNB and SeNB independently perform some of the operations and procedures related to the UE in dual connectivity. For example MeNB can only activate and deactivate serving cells (e.g. SCells) associated with MeNB. SeNB can only activate and deactivate serving cells (e.g. SCells) associated with SeNB. Cross-eNB activation/deactivation is not supported.
The configuration and simultaneous activation, as well as release (hence deactivation), of Special SCell also called as Primary Secondary Cell (PSCell) belonging to SeNB is done by MeNB, and hence that the above mentioned agreement shall only refer to SCells associated with MCG and SCG, respectively. The PSCell is responsible for performing operations within SCG for example operations such as configuration, deconfiguration, activation and deactivation of SCell(s) in SCG. Hence, for example, the MeNB configures and activates the Special SCell but not any of the ordinary SCells in the SCG. Similarly the MeNB deactivates and releases the Special SCell but not any of the ordinary SCells in the SCG.
For configuration and simultaneous implicit activation of Special SCell, it shall be noted that the activation time may be considerable longer than currently assumed for legacy CA. The fact that the Special SCell goes directly into activated state upon configuration means that the UE might not have had a chance to identify it before the activation, hence the activation might be blind. Moreover, as suggested in 3GPP (R2-141849) the UE will also have to acquire SFN timing difference to MeNB by reading MIB from the Special SCell as part of the activation procedure, for purpose of aligning e.g. DRX cycle offset and measurement gap offset between MeNB and SeNB. Acquiring SFN adds an extra 50 ms to the activation time (see analysis in R4-142726) both for regular and blind activation of the Special SCell.
For legacy CA (i.e. CA without dual connectivity) the SCell activation times are 24 and 34 ms for regular and blind activation, respectively; 3GPP TS 36.133 section 7.7. For those numbers to apply it is assumed that the SCell has already been configured by the network via RRC Connection Reconfiguration message (3GPP TS 36.331 section 5.3.5) when the MAC control element activating the cell is received (3GPP TS 36.321 section 5.13). Hence for simultaneous configuration and activation also the RRC procedure delay needs to be taken into account—often 15 ms is assumed for such delay.
Blind activation in legacy CA can make use of that it is known that the maximum time difference between any two cells being aggregated shall be within 30.26 ms (3GPP TS 36.300 annex J.1). Hence the UE only has to assume that the cell to be detected is misaligned by at most half an OFDM symbol, which significantly improves and speeds up the cell detection. In case of unsynchronized MeNB and SeNB both with respect to SFN and frame timing, the UE cannot make such assumption, and the cell detection will be similar to cell detection time for blind handover, which under favourable signal conditions is specified to 80 ms (3GPP TS 36.133 section 5.1).